Exemplary embodiments relate to a method of operating a nonvolatile memory device.
In recent years, there has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased, and which do not require the refresh function of rewriting data at specific intervals.
A read operation and a verification operation of the nonvolatile memory device are performed in a similar way. First, in the state in which a bit line coupled to a selected memory cell to be read is precharged to a high voltage level, a reference voltage is supplied to a word line coupled to the selected memory cell, and the remaining unselected memory cells are maintained in a turn-on state. If the selected memory cell has a threshold voltage having the reference voltage or more, it is in a turn-off state, and so the voltage level of the bit line is not changed. However, if the selected memory cell has a threshold voltage less than the reference voltage, it is in a turn-on state, and the voltage of the bit line is discharged through a common source line in a ground voltage state. Moreover, whether a memory cell to be read has a threshold voltage having a reference voltage or more can be determined based on a shift in the voltage level of a bit line.
According to the read operation or the verification operation described above, if a memory cell, coupled to an unselected bit line, from among memory cells to which read and verification reference voltages are supplied, has a threshold voltage having the reference voltage or more, the corresponding memory cell is turned off, and so two channels with different electrical properties are formed on the basis of the corresponding memory cell. Thus, a voltage of a channel, which is formed in memory cells coupled to a variable voltage terminal of a ground voltage state, is maintained at 0 V. However, a channel of memory cells, coupled between a memory cell of a turned-off state and a source select transistor of a turned-off state, is in a floating state. Accordingly, there is a possibility that threshold voltages of the corresponding memory cells may rise because of voltage supplied to their gates.
A high pass voltage is supplied to the gates of the memory cells. Thus, charges can move to the floating gates of the memory cells because of a hot carrier injection (HCI) phenomenon, leading to a rise in the threshold voltages of the memory cells. In particular, in the case in which the memory cells are in an erase state, such a phenomenon may become worse. Such a phenomenon is referred to as disturbance occurring during the read or verification operation.
The disturbance phenomenon occurring during the read or verification operation results in a change in the voltage level of the bit line. Accordingly, there may be a problem in that the memory cell is measured as having been programmed with a threshold voltage that is less than a target threshold voltage, when in actuality it has been programmed with a threshold voltage that is more than the target threshold voltage. In particular, in the verification operation, an over-program problem (i.e., where a memory cell is programmed with a threshold voltage higher than a target voltage) or an under-program problem (i.e., where a memory cell is programmed with a threshold voltage less than a target voltage) can occur because of the erroneous measurement.